Charging Nickel-metal-hydride
Charging Nickel-metal-hydride
The charge algorithm for NiMH is similar to NiCd with the exception that NiMH is more complex. The NDV method to measure full charge shows only a faint voltage drop, especially when charging at less than 0.5C. A hot battery or one with mismatched cells works against the already minute voltage drop.
The NDV in a NiMH charger must respond to a voltage drop of 5mV per cell. To assure reliable charging, NiMH chargers must include electronic filtering to compensate for noise and voltage fluctuations induced by the battery and the charger. Modern chargers achieve this by combining NDV, voltage plateau, delta temperature (dT/dt), temperature threshold and time-out timers into the full-charge detection algorithm. These “or-gates” utilize whatever comes first depending on battery condition. Many chargers include a 30-minute topping charge of 0.1C to add a few percentage points of extra charge.
Some advanced chargers apply an initial fast charge of 1C. When reaching a certain voltage threshold, a rest of a few minutes is added, allowing the battery to cool down. The charge continues at a lower current and applies further current reductions as the charge progresses. This scheme continues until the battery is fully charged. Known asthe “step-differential charge,” this method works well for all nickel-based batteries.
Chargers utilizing the step-differential or other aggressive charge methods achieve a capacity gain of about six percent over a more basic charger, an increase that is not possible without stressful overcharge. Although a higher capacity is desirable, filling the battery to the brim has a negative effect in that it will shorten the overall battery life. Rather than achieving the expected 350 to 400 service cycles, the aggressive charger might exhaust the pack after 300 cycles.
NiMH cannot absorb overcharge well and the trickle charge is set to around 0.05C. In comparison, the older NiCd charger trickle charges at 0.1C, double that rate. Differences in trickle charge and the need for a more sensitive full-charge detection render the original NiCd charger unsuitable for NiMH batteries. A NiMH in a NiCd charger would overheat, but a NiCd in a NiMH charger does well because the lower trickle charge is also sufficient for NiCd.
It is difficult, if not impossible, to slow-charge a NiMH battery. At a C‑rate of 0.1 to 0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full-charge state accurately and the charger must depend on a timer. Harmful overcharge will occur if a fixed timer controls the charge. This is especially apparent when charging partially or fully charged batteries.
The same scenario occurs if the battery has lost capacity due to aging and can only hold half the capacity. In essence, this battery has electrically shrunk to half size while the fixed timer is programmed to apply a 100 percent charge without regard for the battery condition. In most cases an overcharge will heat the battery, but this is not always the case. A poorly designed NiMH charger is capable of overcharging a battery without heat buildup. At a sufficiently low charge rate, NiMH can remain totally cool and yet suffer from overcharge.
Battery users are often dissatisfied with shorter than expected service life of a battery. Let’s not blame the battery manufacturer, because the fault might lie in the charger. Low-priced consumer chargers are especially prone to incorrectly charging. If you use such a charger and want to improve battery performance, estimate the battery state-of-charge and capacity and set the charge time accordingly. Remove the batteries when you think they are full. If your charger charges at a high charge rate, do a temperature touch. Lukewarm indicates that the batteries may be full enough for removal. It is far better to remove the batteries and then recharge before use than to leave them in the charger for eventual use.
Note that some nickel-based batteries heat up during charge and this is in part due to high internal resistance. Such a battery may be warm to the touch even though it is only partially charged. Another battery might be fully charged and remain cool. Charging NiCd and NiMH batteries on an inexpensive charger is guesswork, and it is best not to charge them longer than necessary. Remove the batteries when perceived full and give them a quick charge before use.
Simple Guidelines on Charging Nickel-based Batteries
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Consumer chargers do not always terminate the charge correctly. Remove the batteries when perceived full.
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Do not charge at high or freezing temperatures. Room temperature is best. Read more about Charging at High and Low Temperatures.
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Do not use chargers that cook batteries. If no alternative exists, charge under supervision and remove the battery when warm to the touch.
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Nickel-based batteries are best fast charged; a lingering slow charge causes memory.
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Below 70 percent, the charge efficiency of an industrial NiMH is close to 100 percent; the battery the pack remains cool.
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Not all NiMH can be charged fast. Excess heat buildup-up is caused by a charge current that is too high, overcharge or and aging battery. Read more about Ultra-fast Chargers.
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A NiMH charger can charge NiCd, but not the other way around. The original NiCd chargers would overcharge NiMH.
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To charge NiMH with a NiCd charger, estimate the time and disconnect the charger manually. Do not leave NiMH on charge longer than needed (full charge detection may not work and the trickle charge current is too high for NiMH).
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Do not leave a nickel-based battery in the charger for more than a few days, even with correct trickle charge. Remove and apply a brief charge before use.
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Nickel- and lithium-based batteries require different charge algorithms. Unless provisions are made, these two chemistries cannot share the same charger.
Comments
And this charging page for NiMH tells you everything BUT the charging of NiMH… It only tells you what C-rate you SHOULDNT charge your cells with - not the recommended C-rate.
So.. what is the recommended C-rate for charging NiMH???
the best C-rate for chargeing NiMH is 0.2C-rate charge 6Hours
Dear Isidor,
I have a question regarding household appliances with NiMH batteries. I own a 18V Black and Decker pv1825n Dustbuster. I found the batterypack details here: http://service.blackanddecker.co.uk/Products/ProductPartsBDK.aspx?ProductID=8339
The factory charger is 7.2-24V voltage converter (85mA). Charge time is around 21hours.
The manual recommends leaving the machine connected to the charger when not in use. I am not sure that would be the correct way to improve lifespan. Regularly, I use the machine every two to three day for a few minutes. From what i have learned off your site i guess the following would be better: charge for 24h, disconnect from charger, use regularly untill I notice capacity starts to drop, charge again for 24h etc. Additionalym a deep discharge every three to four months is a recommended.
What would be your opinion in the case? I find that a lot of appliances come with batteries and instructions that i cannot match well with the information on your site.
My compliments on an excellent website! It has helped me on many occasions and I have made myself an expert in the eyes of family and friends with battery problems (there are many) just by relaying the information i found here. Many thanks!
I am the battery engineer from one of the leading rechargeable battery company
Firstly ,the charge time is 21hours is not reasonable , it is too long time, when the battery is fully discharged ,you must to wait too long for the sencond discharge
Secondly,When the machine not in use for long time such one day or longer, you’d better disconnenct the charger , it will be better for the battery cyclelife. as you know overcharge will be a waste of electricity and will short the battery use life.
Many household appliances supplier worry about that the user can not fully charge the battery which leading to the short run time and complaint .due to the self-discharge of ni-mh battery, in fact , the self-discharge in a short time such as one day or two days is neglectful, and long time overcharge with trickle will be bad for the battery life
So i agree your method to charge and discharge the battery
welcome to discuss this issue
I work on a government base where rules are strickly followed concerning safety. My NiMH batteries for my e-bike are in 15 lb packs of sealed f-cells. (by design 24v, run in series) Do NiMH batteries give off any gasses while charging? What hazzards exist when charging the batteries? (Such as fire, leaking, (I know that Lead acid batteries do and my boss is wanting to wrap me into the same category, leaving me to prove that the NiMH batteries are safe. I need manufacturer documentation to prove this, but my batteries are generic. ON a seperate note, if I charge my batteries in a refrigerator to reduce their heat during charging, I notice that they charge faster. I also notice if I charge them to full then reset the charger after a eight hour full charge that my batteries get an extra 1-3V overall. (normally I am at 57 and if I reset I get upwards of 59.5 after another hour and a half of charging.) Am I hurting the batteries using either process?
there is a safety venting in the battery cap, which will open to give off gasses to release the battery inner pressure when the battery is deep over charged,so, do not deep over charge the battery to long time ,it will lead to leaking even fire or explode
the charge efficiency is related to the temperature, the higher temperature the lower charge efficiency , so, you can charge the battery faster in the refrigerator to reduce their heat during charging. we advise not to reset the charger when the battery is fully charged
to prevent from deep over charged
I work on a R&D institute in Brazil and we’re performing an analisys on NiMH battery for UPS system (aprox. 150W@12V). In this case battery will be constantly charging. Even if I use the proper trickle charge is there a risk of deep over charge?
If the trickle charge is between 0.03C and 0.05 C, there is little risk of deep over charge , However , If you can control the trickle charge every day for one hour to keep the battery fully charged, that will be very good for the battery life.
I have a 20mA constant current source charging a 9V 180 mAHr NiMh battery with a microprocessor A/D that measures the terminal voltage. When the terminal voltage reaches 9.0 software shuts off the charger. The time to reach 9.0 volts from a pretty much discharged state is about an hour so my hunch is that the battery is no where near fully charged but the terminal voltage really never increases much past that level. Can the can the state of charge be determined from terminal voltage alone? I’ve seen tables on flooded lead-acid batteries that relate voltage to percent of charge, but never on NiMh. Is there such a thing? -
The Lead-acid battery is different with Ni-mh battery. As we know, For ni-mh battery ,the fully charged voltage will become higher after some cycles. So, the 9V 180 mAHr NiMh battery can be fully charged controled by the 9.0 volts terminal voltage ,and 20mA when the ni-mh battery is new .but, after some cycles the fully charged voltage will be higher than 9.0V.
We can control the charge by many condition, such as /time, -deltV dt/dT /T.max/ Volt. max
and so on
thank you
I have “old” large capacity 12v NiMH modules (100AH) which I need to keep above 9vdc. I read that trickle charging is not recommended. What would be the recommended “top-up” rate ?
The statement that NICd chargers aren’t good for Ni-MH cells does not take into account the four-fold increase in cell capacity that took place over the time period where Cadmium cells were displaced by metal hydride cells. Consumer trickle-chargers for NiCd batteries do not adjust themselves for the capacity of the cell. The old trickle chargers I had for 600mAH NiCd cells might not be well suited for 600mAH Ni-MH cells, but AA cells in Ni-MH are now 2,000 to 3,000 mAH, so the fast trickle charger that pumped 0.2C/h into a 600mAH NiCd cell is now a slow trickle 0.05C/hr charger, even though the 120mA current flow has not changed. I still use my 1980s vintage NiCd chargers with good (but not optimum) results - VooDude
In everything I’ve read it seems that the worst enemy of Ni-MH is over-charging. My interest is in the long term health of the cell. Would the cell suffer any negative side affects if it was repeatedly charged to only 90% of charge each cycle, maybe by charging at .2C and terminating when deltaT just starts to increase.
Thanks
there is no any negative side affects if it was repeatedly charged to only 90% of charge each cycle at 0.2C , But the work time will short a little ,i hope you do not mind that
thank you
Cuanto tiempo debo cargar una bateria NiMH 7.2 de 1800mAH y cuanto tiempo debe durar la carga de esta bateria. Gracias
Jim wrote:
“I have a 20mA constant current source charging a 9V 180 mAHr NiMh battery with a microprocessor A/D that measures the terminal voltage. When the terminal voltage reaches 9.0 software shuts off the charger. The time to reach 9.0 volts from a pretty much discharged state is about an hour so my hunch is that the battery is no where near fully charged but the terminal voltage really never increases much past that level.”
Jim you don’t have a (nominal) 9V NiMH battery, as the nominal voltage will be a multiple of 1.2. So you have a NiMH in the form factor of a 9V (PP3) battery, but the actual nominal voltage will most likely be either 7.2 volts or 8.4 volts.
To determine the correct final voltage it’s important that you know the number of cells (nominal voltage). If it’s not marked on the battery then measure the open circuit voltage of the battery when it’s getting close to (but not quite) discharged, and see if it’s closer to 7.2 or to 8.4.
My 8.4V 180mA (9 volt PP3 replacements) reach a final voltage of around 10 volts at a 0.1C (18mA) charge rate. A final voltage setting of 9V will be too low for an 8.4V nominal NiMH to charge more than about 20% maximum.
Also be aware that the “correct” final voltage will change with temperature as well as with battery age/condition, so it’s best to have a time limit to the charge as well.
hi,
NiMH batteries are new for me and I want to find some tips for charging and preserving them,I announce you that I have 4 AA batteries Energizer of 2500 mAh and 4 Sanyo 2700 mAh normal (not LSD) and 4-channel smart charger that charge all batteries fully discharged in 3 hours:
1. keeping the battery empty for a week or longer is damaging it ?
2. if the battery is at 70% of capacity and I begin charging , is this damaging it ?
3. how to prevent self discharge as much as possible
4. what is the best use on low temperatures (sometimes if I am outside and is cold my camera wont want even to start and if I come inside the batteries restores and the camera is working ; is there a way to make the camera work at that low temperatures (0 C) not fully charged ? )
5. the charge cycle of NiMh is the same as charge cycle of Li-Ion (0% to 100% or 2 times 50% to 100% etc .)?
thank you !
1. if the battery voltage is above 1.2V, you will not damage the battery even keenp them empty over one mouth.
2. The charger can auto-stop the charge process is the battery is fully charged ,So do not worry about
3.we advise to keep the battery in cool to prevent self discharge
4.low temperature will make the battery work unnormally, We advise you warm the battery before use
5.yes there is no diffenence
if the battery is at 70% of capacity and I begin charging , is this damaging it ? ( same question, George ) 
Hi
I have a customised NiMH battery of 12V 18Ah (2 x 5 M size NiMH cells) and it is charged constantly 8 hours a day by an alternater. We realise that contant charging is not good for it so we want to get a battery charging card which trickle charges it. Would you be able to recommend some battery charging cards which would be suitable for this battery?
hi I have 4 nimh AA batteries of 2500mah (use for my camera) and those have a problem : until last week I charge them with a smart charger that have only 2 chanels ...with -deltaV protection , hight temperature protection..and timer protection ...
recently I bought a smarter charger with 4 chanels and damaged battery protection , and it sems that 3 batteries are damaged (and this charger don’t charge them) ...
how can I restore them (I know that is posible with an analyzer but I don’t afford one)
thank you !
I am not sure if the batteries are damaged or the charger do not charge the batteries because you damaged the protection? I advise you to charge the batteries with the 2chanels (the 2 chanels charger is work well) if the batteries can be fully charged , that means the batteries are OK and the 4chanels charger is damaged.
any enquiry about ni-mh battery, send the email to zzrm316@163.com please
When you need the imformation about ni-mh batteries ,Do Not forget Edward from China
thank you Edward but not the charger is the problem because I have another new more advanced Charger fomam SAnyo enelop (mqr06) and the same problem ; I have also another 8 working batteries , those it charges with all my three chargers ;
I also have a question : can I put in my camera batteries with different capacity ? ex 2 of 2500mAh and 2 of 2700mAh
Dear friend
You’d better not to use the different capacity or different brand rechargeable batteries in the unit at the same time . which maybe lead to over discharge and over charge to the rechargeable betteries.
As you know overdischarge and over charge will damage the battery cycle life.
Edward
zzrm316@163.com
rechargeable battery boy
I was charging AAA cell on a dumb charger that charges at 0.05C (I checked this, 35mA for a 900 mAh cell). When I connected the battery the voltage was 1.37V, but after only a few hours the voltage got up to 1.63V. Is this bad? What is the maximum allowed voltage for NiMH cells and should the charger have some sort of voltage control?
Firstly, I do not think the charger you chose is reasonable for the 900mAh battery. It will take too long time to fully charge the battery—30 hours at least !!it is normal that the ni-mh battery voltage up to 1.63V when it is connect with the charger .and it is fully charged when the voltage up to 1.63V if the battery is OK.you can send some battery and charger photos to my Email for my detail analysis
Edward
zzrm316@163.com rechargeable battery boy
That cell is cheap low quality Chinese or Turkish (brand C.F.L.) cell that certainly doesn’t have 900 mAh. My MP3 player that draws about 90-110 mA only runs for about 1h45m on that cell, but it doesn’t fully discharge it. After it shuts down saying that the battery is flat, the cell still has about 1.18V when loaded with 5 ohm resistor. I never bothered to measure the capacity accurately (I’d have to use a stopwatch for that because I don’t have battery analyzer), but it’s safe to say that it is half of the rated value, so 35 mA is reasonable current. Sorry I forgot to mention all this in first post.
The “charger” is a simple wall plug with open circuit voltage of about 6V and a resistor to limit the current. I terminate the charge when I estimate that the battery is full. I also charge two or three cells in series on this charger with a lower resistor value.
I’ve read several articles on recharging NiMH and they all say that slow charge at about 0.1C (or 0.05C) is OK, but none of those articles mention the voltage. Can these cells be damaged by overvoltage even if the current is kept low and what is the maximum safe voltage for NiMH cell? Should the slow charge be terminated when voltage reaches certain point?
I am working on a school project involving charging 4-AAA NiMH batteries, 1.2V each, with a small solar panel used for outside lighting. The solar panel being used is a 12V 40mA solar panel. There are restrcitions on the size of the solar panel that can be used. The batteries are charging in series. Would it be better to use a solar panel that had a lower voltage rating and higher current rating, even if the to total output power of the solar panel is less. I was thinking that an incease in current would cause the batteries to charge faster
different charge current need different max voltage to cut off the charge. generally speaking. the max. voltage should not more than 1.5V when charged by 0.1c and not more than 1.56V when charged by 1C. if you have more details to know ,please send to my email
zzrm316@163.com
Edward
i read manual from my B65A+ multi charger (made in china) allowable fast charge current is 1C-2C (depend on performance of cell) and cut off discharge voltage is 1.0V/cell. My question is
1. how we going to know which battery use with which charge current.(ex. 0.2C, 0.5C, 1C or 2C)
2. if my battery capacity is 1800mAh and i,m going to charge with 1C. Is it mean my current setting shall (1*1800 = 1.8 A).
3. if the value above (1C-2C, cut off 1.0V/cell) wrong, so what are the value actually
Thanks
8.4v PP3 type 170mAh NiMH back-up battery within an intruder alarm system. Since the mains supply is very reliable (to date!), the battery is almost never utilised. It’s currently kept on float charge via a 560 ohm resistor from a regulated 9.6v supply. Since I read that a trickle charge rate of C/300 is recommended, would a higher value resistor help to increase the life of the battery? If so, what value should that series resistor be?
please i need information about the white powder released ffrm nickel mh batteries within next 12 hors before 3pm 4th may 2011..please tell me what that powder chemical is and about crystalline or amorphous..